1. Technical Field of the Invention
The present invention relates to a noise suppression device and a noise suppression method.
2. Description of the Related Art
A variety of audio devices have been developed, including an audio playback device that drives load such as speakers or the like according to an input signal, a voice communication device used for voice communication between remote places, and a voice recognition device that determines the meaning of voice by identifying and recognizing the type or the like of the voice. To allow each of these devices to correctly reproduce, transmit, or recognize audio, it is preferable to remove the influence of noise contained in the audio.
Examples of such noise suppression technologies include those described in the following Patent References 1 and 2 and Non-Patent References 1 and 2.    [Patent Reference 1] Patent Application Publication No. 2006-126859    [Patent Reference 2] U.S. Pat. No. 5,572,621    [Non-Patent Reference 1] Boll, S., “Suppression of acoustic noise in speech using spectral subtraction”, IEEE Trans. Vol. ASSP-27, No. 2, pp. 113-120, 1979.    [Non-Patent Reference 2] Doblinger G., “Computationally efficient speech enhancement by spectral minima tracking in subbands”, Proc. of Eurospeech EUROSPEECH'95, pp. 1513-1516.
Technologies described in these references relate to a so-called spectral subtraction method in which noise is suppressed through subtraction from the level of an amplitude spectrum of the frequency domain of input signal using an appropriate technique. These technologies all achieve certain noise suppression effects.
However, these references have undisclosed or unsolved problems.
For example, the spectral suppression method is based on a scheme in which a noise spectrum contained in an input signal is estimated and the noise spectrum estimation value is subtracted from an amplitude spectrum. In this case, there is a problem in that it is difficult to secure the accuracy of the noise spectrum estimation value. That is, since the noise spectrum estimation value is just an estimation, it does not necessarily reflect the actual noise spectrum. As a result, suppression may be performed insufficiently at positions at which more suppression should be performed or, conversely, suppression may be performed excessively at positions at which suppression should not be performed. Thus, the spectral suppression method has a problem in that it is not possible to achieve correct noise suppression.
Patent Reference 1 suggests a method which detects whether or not audio is contained in an input signal and estimates the average spectrum of an interval including no audio as a noise spectrum. Patent Reference 2 and Non-Patent Reference 2 suggest a method in which a noise spectrum is estimated directly from an input signal without performing audio detection.
However, basically, none of the references provide a means for certainly overcoming the above problems since the noise spectrum is still just an “estimation” in all of the references.
In addition, each of the references individually has the following problems. First, in Patent Reference 2, a minimum value is found among values obtained by smoothing the spectrum of an input signal within a limited time window and the found minimum value is used as a noise spectrum. However, this method has a problem in that the capacity of storage for storing the spectrum is relatively large and the amount of calculation for obtaining the minimum value is also great.
In Patent Reference 2, since a noise spectrum is estimated using the minimum value, there is a tendency to underestimate a noise spectrum value in the case where the input signal includes only noise. For the same reason, in the case where the input signal includes an audio signal, there is a tendency to overestimate a noise spectrum value even though the minimum value is used. In the former case, noise that is little suppressed is output and, in the latter case, there is a problem of sound quality degradation due to excessive subtraction of noise spectrum. Non-Patent Reference 2 also has these problems.
Patent Reference 1 has a problem in that it is not necessarily possible to cope well with the case where the level of noise rapidly changes. That is, in Patent Reference 1, in the case where the level of the input signal has rapidly changed, the noise spectrum estimation value is not updated, assuming that the signal should have contained audio. However, there is a problem if the rapidly changed level is not associated with audio (namely, signal), but associated with a noise. If this situation continues, the noise spectrum estimation value is also not updated, causing a problem in that noise with the rapidly changed level is retained and reproduced.